using System;

​

    public static class Extensions

    {

        public static bool IsEmptyOrNull(this string str)

        {

            return string.IsNullOrEmpty(str);

        }

    }

​

public class Program

{

    public class Zone

    {

        public string PublicId { get; set; }

        public string ParkingCode { get; set; }

    }   

​

    public static void Main()

    {

        var zone = new Zone(){PublicId = "fu", ParkingCode = "bar"};        

        var outcome = zone?.PublicId.IsEmptyOrNull() ?? false ? zone?.ParkingCode : zone?.PublicId;     

        Console.WriteLine(outcome);

        zone = new Zone(){PublicId = null, ParkingCode = "bar"};        

        outcome = zone?.PublicId.IsEmptyOrNull() ?? false ? zone?.ParkingCode : zone?.PublicId;     

        Console.WriteLine(outcome);

    }

}